Gradient in Blender Render
You can use the blend texture for this.

Go to the textures tab in the Properties Editor. In the material sub-tab (it's open by default), create a new Blend texture.



Change the mapping of the texture to "Object", and select the object that represents the model. Set the X coordinate input to Z (because we want the gradient to be vertical). The other settings don't matter, because the Y and Z coordinate inputs don't affect the output.



Now that the texture is mapped correctly, you can change it's influence (in the influence drop-down accordion). In your case, the texture should influence the color of the diffuse shader. If your material is brown, this texture should make it blonde (yellow-ish) on one end. The color influence is at 1 by default, which is what we want. Just change the color to what you want.



You should now have a basic gradient from blonde to brown. Then, just play around with the mapping and influence (and other) settings, until you get the look you want.